1. Field of the Invention
This invention relates to a central monitoring system that can monitor at a local switch an occurrence of a fault on a subscriber line connected via a subscriber loop carrier to the local switch.
2. Description of the Related Arts
Recently a subscriber loop carrier, generally referred to as SCL, has been popularly employed in a telecommunication system in order to economically accommodate the increasing number of subscriber lines to be connected with a local switch LS. A typical circuit configuration of prior art subscriber loop carrier SLC system is schematically illustrated in FIG. 1.
A plurality of subscriber terminals 2.sub.1 to 2.sub.n are connected via respective first subscriber lines 5 each formed of a copper wire pair, denoted with 2W, to a remote terminal, generally referred to as a RT, 3.
RT 3 is provided with a first multiplexer/demultiplexer 32 which multiplexes/demultiplexes signals on first subscriber lines 5 to/from a carrier line 6 typically modulated by a pulse code modulation, generally referred to as PCM. The other end of carrier line 6 is connected to a second multiplexer/demultiplexer 42' provided in a central office terminal 4', generally referred to as a COT. Second multiplexer/demultiplexer 42' demultiplexes/multiplexes the signals on carrier line 6 to/from second subscriber lines 7, the other ends of which are connected to a local switch 1. Each of second subscriber lines 7 is formed of a pair of two wires.
Thus arranged is a subscriber loop carrier SCL between first subscriber lines 5 and local switch 1.
RT 3 is further provided with a first subscriber line tester 33', which is connected via transfer switches 31.sub.1 to 31.sub.n provided between each of first subscriber lines 5 and first multiplexer/demultiplexer 32, where the suffix 1 to n denote the channel numbers. Each of transfer switches 31.sub.1 to 31.sub.n is installed respectively on interface cards, and usually connects each of the first subscriber line 5 to first multiplexer/demultiplexer 32. In testing first subscriber lines 5 one of transfer switches 5 switches one of first subscriber lines 5.sub.n to the first subscriber line tester 33'.
First subscriber line tester 33' measures DC (direct current) voltage, AC (alternate current) voltage, insulation resistances as well as capacitances of each subscriber line. These measured values are respectively compared with predetermined specification values shown in FIG. 2 in order to judge whether the subscriber line is below or over the specification values, i.e. normal or faulty.
When a fault is detected by first subscriber line tester 33' a fault signal informing the data of the fault is transmitted from subscriber line tester 33' via a data link line 8 provided in carrier line 5 to a control console 40 provided in COT 4'.
In COT 4', between each second subscriber line 7.sub.n and multiplexer/demultiplexer 42' there are provided interface cards 39 for connecting each channel output of multiplexer/demultiplexer 42 to each of second subscriber lines 7. Control console 40 controls via data link line 8 the subscriber line tester 33' in RT 3 to test both the subscriber terminals 2 and the subscriber lines 5 so as to check whether they are normal or faulty.
In local switch 1, there are provided interface cards for connecting each of the second subscriber lines 7 to the subscriber side inputs of local switch 1. On each interface card there is provided a transfer switch 11.sub.1 to 11.sub.n. A control console 13 controls a second subscriber line tester 12, which tests each of second subscriber lines 7 by being connected to one of second subscriber lines 7 by transfer switch 11.sub.1 to 11.sub.n.
The two subscriber line testers 33' and 12 are thus provided independently from each other, where first control console 40 covers subscriber terminals 2 and first subscriber lines 5, as well as second control console 12 covers second subscriber lines 7.
Problem in this prior art SCL monitor system is in that in order to check the status of the overall subscriber lines both of the subscriber line testers 33' and 12 have to be operated to test the respective coverage, resulting in troublesome manual operations. Control console 13 in local switch 1 can identify second subscriber lines 7 with subscriber's telephone numbers; however, control console 40 in subscriber loop carrier SLC must identify the first subscriber lines 5 with channel numbers. Therefore, in testing the first subscriber lines the channel number must correspond with the respective telephone number, resulting in a cause of troublesome procedure.